1. Field of the Invention
The present invention relates to a method of producing a semiconductor device such as a thin film transistor (TFT), and more particularly to a method of producing a semiconductor device using a crystalline silicon film on a substrate having an insulating surface.
2. Description of the Related Art
In recent years, attention has been paid to a technique by which a thin film transistor (TFT) is formed using a silicon thin film formed on a glass substrate. The TFT is primarily used for a liquid crystal electro-optical device of the active matrix type. Also, the TFT is used for a variety of thin film integrated circuits.
The liquid crystal electro-optical device is designed such that liquid crystal is sealingly interposed between a pair of glass substrates, and the optical characteristic of the liquid crystal is changed by the application of an electric field to the liquid crystal, to display an image.
The liquid crystal display unit of the active matrix type for which the TFT is used is characterized in that a TFT is arranged in each pixel, and the charges stored by a pixel electrode are controlled by the TFT that functions as a switch. The liquid crystal display unit of the active matrix type is used as a display for various electronic devices (for example, a portable word processor or a portable computer) because it is capable of displaying a fine image at high speed.
The TFT used in the liquid crystal display unit of the active matrix type is generally produced on a transparent substrate such as a quartz substrate or a glass substrate and uses an amorphous silicon thin film. However, the TFT using the amorphous silicon thin film has problems stated below.
(1) The characteristic is low, and image display with higher quality cannot be performed.
(2) A peripheral circuit for driving a TFT disposed in each pixel cannot be constructed.
As a method for solving the problems (1) and (2), there is a technique for producing the TFT using a crystalline silicon thin film. As a method for obtaining the crystalline silicon thin film, after the amorphous silicon film is formed on a glass substrate or a quartz substrate by plasma CVD or low pressure CVD, there is a method that an amorphous silicon film is thermally treated, and a method that a laser beam is irradiated onto an amorphous silicon film.
The problem (2) is considered to be classified into such a problem that a CMOS circuit cannot be constructed since the TFT using the amorphous silicon thin film does not put a p-channel type TFT into practical use, and such a problem that the peripheral drive circuit cannot be assembled since the TFT using the amorphous silicon thin film cannot operate at high speed and also a large current cannot flow therein.
As a method in which the amorphous silicon film is crystallized by a heat treatment, there has been known a structure disclosed in Japanese Patent Unexamined Publication No. 6-232069. This method enables a crystalline silicon film to be obtained under a condition in which a heat treatment is conducted at 550.degree. C. for 4 hours, using a metal element such as nickel which promotes the crystallization of silicon. Hence, the crystalline silicon film can be formed even on a glass substrate lower in heat resistance than a quartz substrate.
However, according to the above publication, there arises an unsatisfactory matter in the crystallinity of a crystalline silicon film to be obtained. In other words, the crystallinity of the crystalline silicon film to be obtained is low, which causes a large amount of amorphous silicon components to remain.
Also, if the heat treatment conditions are 550.degree. C. and 4 hours, the crystalline silicon film can be formed at a level where no problem is caused by distortion or deformation of a glass substrate of about 10 inches in size, using a metal element such as nickel. However, there is a demand that the liquid crystal display unit is increased in area, and it is expected that a liquid crystal display device of 20 inches, further 30 inches or more diagonally will be produced. In such a large area, the distortion or deformation of the glass substrate causes a problem even by the technique disclosed in the Japanese Patent Unexamined Publication No. 6-232069 is used.
The liquid crystal electro-optical device is so designed as to interpose a liquid crystal between a pair of glass substrates with an interval of several .mu.m. Thus, the distortion of .mu.m order between the edges of the substrate causes the thickness of a liquid crystal layer to be changed. This causes the nonuniformity of display and so on. Also, in producing a semiconductor integrated circuit formed on a glass substrate, the problem leads to the lowering of a yield accompanied by the failure of exposure or the failure of substrate transportation.
In the Japanese Patent Unexamined Publication No. 6-232069, there is observed such a phenomenon that a metal element such as nickel as used is locally concentrated in the crystalline silicon film. This phenomenon comes to a factor that leads to a defect when constituting the device. Also, this becomes a factor that lowers the yield of production.
Further, as a technique in which an amorphous silicon film is transformed into a crystalline silicon film, there has been known a technique in which a laser beam is irradiated onto the amorphous silicon film. The irradiation of a laser beam enables the crystalline silicon film partially high in crystallinity to be obtained. However, it is difficult to obtain the effect caused by the irradiation of a laser beam with high reproducibility, and also it is difficult to obtain the crystalline silicon film over a large area.